1. Field of the Invention
The present invention relates to a display apparatus using a liquid crystal, particularly a ferroelectric liquid crystal, which can be driven by an active matrix driving method to perform gradation display, i.e., the display of a natural dynamic picture.
2. Description of the Related Art
A ferroelectric liquid crystal display apparatus capable of performing gradation display is disclosed in Japanese Patent Laid-Open No. 63-249897 corresponding to U.S. Pat. No. 4,840,462 and EP 284,134.
Briefly, the operation of this type of display apparatus is as follows. As the first step, each pixel of the matrix is reset to a first stable state. Then, an electrostatic charge Q is imparted to the pixel electrode through an active element so as to cause a switching of the ferroelectric liquid crystal into a second stable state in a local portion of each pixel. According to this operation principle, representing the area of the local portion switched to the second stable state by a and the magnitude of spontaneous polarization of the ferroelectric liquid crystal by P.sub.S, charges of an amount expressed by 2P.sub.S .multidot.a are moved and this movement, i.e., the switching to the second stable state, continues until the charge amount 2P.sub.S .multidot.a grows to a level large enough to cancel the initially given charge Q. Finally, a portion of each pixel having the area a represented by the following formula (1) is switched to the second stable state. EQU a=Q/2P.sub.S ( 1)
It is therefore possible to control the area a by varying the charge amount Q, thus attaining an area gradation.
In general, this type of display apparatus is used in such a manner that the optical axis in the first stable state is aligned with one of the axes under a cross-nicol polarizer so that the first and second stable states respectively provide a black display and a white display.
In the known ferroelectric liquid crystal display apparatus of the type described, the gradation display (area gradation control) is performed by controlling the inversion domain area by canceling, by the inversion of spontaneous polarization of the ferroelectric liquid crystal, the charge which has been accumulated in each liquid crystal pixel which is regarded as being a capacitor. In this type of display apparatus, therefore, it is essential that each pixel not have any sub-capacitance.
This requirement, however, is not actually met. Referring to FIG. 5, a TFT (Thin Film Transistor) is used as the active switching element. Such element has parasitic capacitances C.sub.gd between the gate and the drain and C.sub.sd between the source and the drain. Consequently, noises are introduced into the pixel electrode through these parasitic capacitances. FIG. 6 shows, by way of example, the influence on the pixel electrode voltage (voltage applied to liquid crystal layer) V.sub.LC caused by gate pulse noise which is introduced through the parasitic capacitance C.sub.gd, on the assumption that there is no sub-capacitance. It will be seen that the pixel electrode voltage is reduced by .DELTA.V.sub.1 immediately after application of a reset voltage V.sub.R, by the influence of falling noise of the gate pulse. Similarly, the pixel electrode voltage is lowered by .DELTA.V.sub.2 immediately after the application of the writing voltage V.sub.W. These phenomena are generally referred to as "oscillation by parasitic capacitance" and the degree of influence of such phenomena is determined by the ratio between the parasitic capacitance and the capacitance of C.sub.LC of the liquid crystal. The degree of influence also becomes greater as the density of the pixels is increased. The oscillation by parasitic capacitance, therefore, adversely affects the display and, in the worst case, makes it impossible to effect the desired display.
It has generally been known to provide a sub-capacitance C.sub.S in parallel with the liquid crystal pixel capacitance C.sub.LC as shown in FIG. 2, in order to reduce the oscillation by parasitic capacitance in liquid crystal devices which operate in an on-off mode, the capacitance of such sub-capacitance being 5 to 10 times as large that of the capacitance C.sub.LC of the liquid crystal. Provision of the parallel sub-capacitance C.sub.S in the manner shown in FIG. 2 poses the following problems. When the cancellation of the charge in the liquid crystal capacitance C.sub.LC by the inversion of ferroelectric liquid crystal is commenced during driving of such ferroelectric liquid crystal, charges are supplied from the parallel sub-capacitor C.sub.S to the liquid crystal capacitance C.sub.LC. Such charges form a current i. As a consequence, the .gamma.-characteristic (voltage/transmissivity characteristic) of the ferroelectric liquid crystal is changed to have a very steep gradient as shown by the broken line in FIG. 4. Consequently, the inversion domain spreads beyond the expected area of inversion, thus hampering the gradation control. The broken-line curve in FIG. 4 shows the .gamma.-characteristic of the ferroelectric liquid crystal as obtained when an auxiliary parasitic capacitance C.sub.S, having a capacitance 5 times as large that of the pixel capacitance C.sub.LC, is connected to a conventional ferroelectric liquid crystal in parallel therewith.